Optional telephone services are features implemented by a switch in a telephone network such as, for example, a DMS-100 or a SONUS Soft Switch. Optional telephone services may include call forward busy, call forward no answer, and call waiting. Optional telephone services are distinguished from voice services, which are services related to the exchange of voice communications that are not implemented by a switch in the telephone network but rather are implemented by other systems within or communicatively coupled to the telephone network. Voice services may include voicemail and call alerts.
Voice services typically use optional telephone services such as call forward busy and call forward no answer to redirect calls across the telephone network to systems implementing the voice services.